Fig. 12. (a) Electron-density peaks over the
adenosyl residue calculated from X-ray data on dimethylbenzimidazole cobamide
coenzyme. The calculation illustrated was one in which phase angles were computed
from the positions of all the atoms in the molecule except those shown. Although
only part of the electron density appears, the relative weights of the atoms are
in agreement with their chemical structure. (b) The atomic positions found for
the coenzyme molecule projected along the crystallographic b axis.